As ignition means for internal combustion engines of, for example, motor vehicles, there are known spark plugs which have a spark discharge gap formed by opposing a center electrode and a ground electrode in an axial direction. Such spark plugs cause a spark discharge in the spark discharge gap, and ignite an air-fuel mixture in a combustion chamber of an internal combustion engine by the spark discharge.
In the combustion chamber, there is formed a gas flow (i.e., a flow of the air-fuel mixture), such as a swirl flow or tumble flow. With the gas flow moderately flowing also in the spark discharge gap, it is possible to ensure the ignition capability of the spark plug.
However, depending on the mounting state of the spark plug to the internal combustion engine, part of the ground electrode, which is joined to a distal end of a housing, may be located upstream of the spark discharge gap with respect to the gas flow. In this case, the gas flow in the combustion chamber may be blocked by the ground electrode; thus the gas flow in the vicinity of the spark discharge gap may stagnate. As a result, the ignition capability of the spark plug may be lowered. That is, there may be a problem that the ignition capability of the spark plug varies depending on the mounting state of the spark plug to the internal combustion engine. In particular, in recent years, lean-burn internal combustion engines have been widely used; in those internal combustion engines, the combustion stability may be lowered depending on the mounting state of the spark plug.
Moreover, it is difficult to control the mounting state of the spark plug to the internal combustion engine, more specifically the mounting position of the ground electrode of the spark plug to the internal combustion engine. This is because the mounting state varies depending on the state of formation of mounting threads in the housing of the spark plug and the fastening degree of the spark plug in the mounting process to the internal combustion engine.
Accordingly, in Patent Document 1, it is proposed to provide an inclined surface in a side surface of the ground electrode so as to allow the gas flow to be guided to the spark discharge gap even when the ground electrode is located upstream of the spark discharge gap. More specifically, in at least one of a pair of side surfaces of the ground electrode, there is provided an inclined surface that is inclined toward the other side surface as it approaches the center electrode. By this, it is aimed to have the gas flow flowing toward the ground electrode (or the spark discharge gap) along the inclined surface by the Coanda effect.